1. Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati Part 10 – Self-assembly at surfaces
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2. Supramolecular systems on surfaces
Simple deposition on a solid surface
Langmuir-Blodgett (LB) films
Self-assembled monolayers (SAM)
Incorporation in membranes
Derivatized metal and metal oxide nanoparticles
Applications
Sensoring
New materials (e.g., molecule-based electronic circuits, display systems, smart windows, ...)
Heterogeneous catalysis
Construction of nanostructures
Molecular-level devices (”micro-nano” problem)
Molecular-level electronic components
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3. The Langmuir-Blodgett technique
An amphiphilic species, dissolved in a volatile organic solvent, is spread onto an aqueous surface.
After evaporation of the organic solvent, the amphiphilic molecules start to reorganize, forming a film in which the nonpolar tails of the amphiphile rise above the aqueous subphase, while the polar head groups remain immersed.
At this point, the molecules are dispersed and the film is not continuous.
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4. The Langmuir-Blodgett technique - 2
The surface is compressed by means of a moving barrier (which is part of the trough). A feedback system, called Wilhelmy plate, controls the position of the barrier, and thus the surface pressure.
Free-floating films on the trough surface are called Langmuir films.
Wilhelmy plate
Moving barrier
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5. The Langmuir-Blodgett technique - 3
Langmuir films can be physically transferred onto solid supports by dipping the support into the film. Deposition of a Langmuir film on the surface of a substrate results in Langmuir-Blodgett films.
Suitable substrates for the preparation of LB films are
Hydrophilic substrates like glass and quartz (for spectroscopic studies)
Metal hydrophilic plates like chromium, aluminium, tin, all in the form of oxides
Conductive substraates like silver and gold (for electrochemical studies: modified electrodes)
Semiconductor plates (silicon, gallium arsenide, ...)
Atomically flat and highly hydrophobic substrates like mica
Monolayer LB films can be deposited on the surface by vertical deposition (VD) in either of two possible orientations, which can be described simply as tails in (X-type, transfer on the downstroke) or heads in (Z-type, transfer on the upstroke).
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6. The Langmuir-Blodgett technique - 4
To obtain information about the stability of the film, the reorientation of the molecules, the phase transitions and conformational transformations, pressure-area isotherms must be studied.
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7. The Langmuir-Blodgett technique - 5
Note that the hydrophilicity of the substrate may influence the ease of preparation of monolayers in a particular orientation. For example, the Z-type deposition is likely to be easier on a hydrophilic substrate.
Subsequent layers may be added by repeated vertical dipping in either mode.
Films deposited in this fashion, while initially organized, are subject to relaxation of the packed film. The organization of the film is degraded after transfer: without the mechanical pressure provided by the floating barrier, the tight packing of the film may be quickly lost.
Important parameters for the preparation of the LB film:
Surface pressure maintained during the transfer;
Transfer (rising or dipping) speed;
Time elapsed between successive monolayer deposition.
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8. The Langmuir-Blodgett technique - 6
To overcome these problems, one can alternatively employ the horizontal touching (HT) technique. In this method, the substrate is gently lowered onto the surface of the trough and the amphiphilic molecules adhere to the substrate in an X (tail in) orientation.
Because the substrate remains in contact with the trough solution, which is typically maintained at a constant pressure, the film deposited on the surface is less subject to relaxation.
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9. LB technique for the fabrication of moleculear devices
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10. Developing molecule-based electronics
Bi-functional amphiphilic rotaxanes are self-organized in a Langmuir-Blodgett film deposited on a SiO2 substrate patterned with preformed poly-Si strips.
An 8x8 cross-bar device is formed between poly-Si electrodes (100 nm wide) at the bottom and Ti/Al electrodes (70 nm wide) on top. Each molecular tunnel junction with an area of 7000 nm2 contains about 5000 molecules.
The device works as a 16-bit memory unit by controlling the voltage at each arm of the cross-bar.
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11. Self-assembled monolayers (SAM)
The term self-assembled monolayers (SAMs) is used to described organized molecular assemblies (one-molecule thick) whose spontaneous formation and stability depend on favorable, if weak, intermolecular forces, as well as on forces between each of the individual component molecules and the solid substrate.
Molecules used to prepare SAMs and LB films differ in that the former must interact chemically with the surface, that is, form chemical bonds, and are soluble in the fluid from which they are prepared.
The most popular type of SAM is that formed by the chemisorption of long-chain alkanethiol molecules on clean gold surfaces. The main driving forces for this self-assembly process are: (i) the formation of thiolate-gold bonds (estimated to release kcal/mol), and (ii) the favorable lateral interactions among the aliphatic chains of neighboring alkanethiol molecules.
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12. SAMs of thiol compounds on Au surfaces
These SAMs are prepared simply by exposing a clean gold surface to a solution of the alkanethiol.
For alkanethiols with chains longer than 10 carbon atoms, it is generally true that:
A stable, organized and densely packed monolayer is readily formed on the Au surface.
The thickness of the monolayer is directly proportional to the length of the alkanethiol chain.
The main axes of individual molecules are tilted by an angle of ca. 30° from the normal to the surface.
Every chemisorbed molecule loses the hydrogen atom from the thiol group. The surface-attached molecules are thus better formulated as thiolates.
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13. Functional SAMs
The figure below illustrates another important feature of SAMs. Synthetic modification of the thiol permits the attachment of functional groups (R) to the monolayer.
Among other applications, this feature can be used to control the wetting properties of the surface, to attach redox-active or photoactive groups at defined distances to the gold surface, and to incorporate binding sites which can lead ultimately to molecular recognition of suitable targets.
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14. Examples of SAMs for sensor applications
Adv. Mater. 2000, 12, 1315
SAMs of b-cyclodextrin and their complexation with ferrocene
Binding of b-cyclodextrin to a dansyl-functionalized monolayer on glass
Binding of crown fullerene to a cystamine monolayer on gold
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Molecular recognition by a fluorescent receptor adsorbate on glass